In my Father's house are many MANSIONS: I go to prepare a place for you.

[TonyChanYT]

NASB 1977 John 14:

2 In My Father’s house are many dwelling places [G3438]; if it were not so, I would have told you; for I go to prepare a place for you.

μονή (G3438) appears in only two verses. John 14:

23 Jesus answered and said to him, “If anyone loves Me, he will follow My word; and My Father will love him, and We will come to him and make Our dwelling [G3438] with him.

Both times, it was used by Jesus. It means an individual dwelling unit. It is the noun form of the verb μένω (G3306) which means to stay, abide, or remain. The verb form appears in the next chapter, John 15 (Berean Literal Bible):

1 I am the true vine and My Father is the vinedresser. 2 Every branch not bearing fruit in Me, He takes it away; and every one bearing fruit, He prunes it that it may bear more fruit. 3Already you are clean by reason of the word that I have spoken to you. 4 abide [G3306] in Me, and I in you. As the branch is not able to bear fruit of itself unless it abides in the vine, so neither you, unless you abide in Me.
5I am the vine; you are the branches. The one abiding in Me and I in him, he bears much fruit. For apart from Me you are able to do nothing. 6 If anyone does not abide in Me, he is thrown out like the branch and is dried up, and they gather them and cast them into the fire, and it is burned. 7 If you abide in Me and My words abide in you, you shall ask whatever you wish, and to you it will come to pass. 8 In this My Father is glorified, that you should bear much fruit, and you shall be My disciples.
9As the Father has loved Me, I also have loved you. Abide in My Love. 10 If you keep My commandments, you will abide in My love, as I have kept My Father’s commandments and abide in His love. 11 These things I have spoken to you, that My joy may be in you and your joy may be full.

John 14:2 says that in the Father's house are many dwelling/abiding places. Jesus went away to prepare the Holy Spirit as Paraclete to come to dwell/abide in us. This abiding relationship begins in the present life and onto eternity. We don't have to wait until after our resurrection to move into these so-called mansions. It is actually the other way around. Presently, God moves into these dwelling places into us believers. Jesus' point is the present abiding places here and now, and not future retirement mansions in heaven